In recent years, increase in integration degree, higher information density, and higher image definition have been desired more and more in fields of optics and electronics. Therefore, a film used in these fields is strongly required to have a finer structure on its surface. Namely, forming a fine pattern structure (hereinafter referred to as fine patterning) has been strongly required. Additionally, in a field of research for a regenerative medicine, a film having a fine structure on its surface is effectively used as a scaffold for cell culture.
Various methods for the fine pattering have been put to practical use. For example, there are a deposition method using a mask, an optical lithography adopting photochemical reaction and polymerization reaction, a laser ablation technique, and the like.
Additionally, as the fine patterning, there is known a method for forming a porous film having a plurality of pores by blowing wet air toward a coating film which is formed from a polymer solution, as disclosed in Japanese Patent Laid-Open Publication No. 2001-157574 or No. 2007-291367, for example. The method for forming a porous film is described roughly hereinbelow. At first, a solution containing a hydrophobic polymer and a hydrophobic solvent, as materials of the porous film, is applied to a support to form a coating film on the support. Then, wet air is blown toward an exposed surface of the coating film, such that water vapor is condensed from ambient air on the exposed surface to generate water drops thereon. The generated water drops are arranged regularly with the support of capillary force and the like. Wet air is further blown toward the exposed surface such that the generated water drops are grown up. After the water drops are grown up to have a desired size, the blowing of the wet air is stopped. Thereafter, a predetermined gas is supplied to the coating film to evaporate the solvent from the coating film. Thereby, fluidity of the solution for forming the coating film is decreased, and as a result, it is possible to obtain a primary body having film form, which has a plurality of pores. Note that, the pores are made by using the water drops as a template of a porous structure. Finally, dry air is blown toward the primary body to evaporate the water drops and the solvent remained in the primary body. Accordingly, the porous film can be obtained.
It is known that the size and formation density of the pores in the porous film obtained by the above method are influenced by the formation amount of cores of the water drops and the growth degree of cores of the water drops during the production process. Additionally, it is known that it is possible to control the formation amount of cores of the water drops and the growth degree of cores of the water drops by arbitrarily adjusting a parameter ΔTw. The parameter ΔTw is obtained by subtracting a temperature TS of the exposed surface of the coating film from a dew point TD of air around the exposed surface of the coating film. Therefore, when the water drops are generated or grown up on the exposed surface under the arbitrarily adjusted parameter ΔTw, it is possible to achieve desired size and formation density of the pores in the porous film as a final product.
In such a method for forming the porous film, the viscosity of the solution for forming the coating film is preferably as low as possible, such that the water drops can be generated, grown up, and arranged regularly on the coating film. In view of the above, it is required to use a solution in which the content of the solvent is extremely larger than that of the hydrophobic polymer in the method for forming the porous film.
In a case where the solution in which the content of the solvent is extremely large is used to produce the porous film as described above, an immense amount of time is required to evaporate most solvent contained in the coating film. Therefore, it has been difficult to improve productivity of the porous film.